This is what I have. Hopefully I got it right because I had to type it out manually from looking at the form rules.
Rule 1: (ActivitySelected = TRUE and checkbox1 is not blank)
Condition: ActivitySelected = TRUE and checkbox1 is not blank
Action: Show dialog message: Only 1 Activity Alarm can be selected at a time!
Rule 2: (ActivitySelected = FALSE and checkbox1 is not blank)
Condition: ActivitySelected = FALSE and checkbox1 is not blank
Action: Show dialog message: Only 1 Activity Alarm can be selected at a time!